1. Field of the Invention
The present invention relates to a method for evaluating and producing a semiconductor film formed on a substrate and an apparatus for producing the semiconductor film, and particularly to a method for evaluating and producing a semiconductor film and an apparatus for producing the semiconductor film being capable of incorporating an in-line evaluator of the semiconductor film thereinto.
2. Description of the Related Art
Active matrix LCDs (Liquid Crystal Displays) have been developed for mass production. The active matrix LCDs are highly integrated circuits whose level of integration is enhanced by using a technique of producing a semiconductor film on a substrate. The active matrix LCDs enable display of animation with high resolution. Such high resolution animation is obtained through a TFT (Thin Film Transistor), which is a switching element of a matrix display portion, disposed on one of two substrates with a liquid crystal interposed between them.
If the TFT can exhibit similar properties to that of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) produced on a silicon substrate and the TFT can be produced on an insulating substrate, not only the switching elements but also peripheral driver circuits for providing desired driving signal voltages can be produced on the same substrate of the matrix display portion by forming a CMOS (Complementary Metal Oxide Semiconductor) around the display portion, whereby TFT-LCD with integrated driver circuits can be mass produced.
The TFT-LCD with needs no externally attached driver IC to a liquid crystal panel, thereby reducing processing steps and narrowing a frame. By narrowing the frame, a recent product such as a portable information terminal monitor and a handy video camera monitor would be small in size.
One of the critical objects in realizing the LCD with driver circuit is to form a good semiconductor film on a transparent insulating substrate such as glass and the like at a temperature within a heat resistance limit of the substrate. Conventionally, an amorphous semiconductor film, especially amorphous silicon (a-Si), is formed at a relatively low temperature of about 300.degree. C. to 400.degree. C. to produce the TFT on the substrate. However, such an a-Si TFT has a high on resistance. Therefore, it can be applied to the switching element on the matrix display portion, but cannot be utilized as a driver circuit requiring faster operation.
In contrast, poly crystal semiconductor includes many single crystal grains having a diameter of hundreds of .ANG. to thousands of .ANG. in contact each other. Once the poly crystal semiconductor is used for a channel layer, the TFT applicable to the driver portion can be formed. Especially a poly crystal silicon, i.e. polysilicon (p-Si) has a mobility of about tens to hundreds cm.sup.2 /V.multidot.s which is two orders of magnitude greater than that of the a-Si. When the p-Si is used, the CMOS is formed to have sufficient speed to be utilized as the driver in the LCD.
When the p-Si TFT LCD with integrated driver circuit is formed, forming the p-Si having a good film property on a glass substrate is most important. The p-Si film is typically formed by a SPC (Solid Phase Crystallization) method that applies heat to the a-Si film formed on the substrate to induce crystallization or by a low pressure CVD (Chemical Vapor Deposition) method that directly deposits the p-Si on the substrate. These film making methods are carried out at a high temperature of 700.degree. C. to 900.degree. C. Such a p-Si TFT LCD producing process is called a high temperature process, because it involves steps carried out at high temperature. In the high temperature process, an expensive substrate such as a heat-resistant quartz glass or the like is required, which leads to a high cost.
To decrease the cost, the applicant has developed a method conducted at a lower temperature of about 600.degree. C. or less using an inexpensive non-alkali glass substrate or the like. Such a process of producing the p-Si TFT LCD while maintaining a critical temperature or less for the heat resistance of the substrate during all processes is called a low temperature process.
The low temperature process can be realized by ELA (excimer laser annealing) in which an excimer laser is applied to the a-Si to produce the p-Si by promoting crystallization. The excimer laser is an ultraviolet light produced when an excited excimer returns to a ground state. In the ELA, a laser beam shape is deformed with a predetermined optical system to irradiate to be processed film. Thermal energy is thus selectively applied to a surface of the a-Si. Crystallization is then carried out at a critical temperature or less for the heat resistance of the substrate to form the p-Si.
In the ELA, major problems are attaining optimum laser power setting and eliminating dispersion of irradiated laser energy. According to a relationship between the irradiated laser energy and a crystal diameter (grain size) of the p-Si as shown in FIG. 1, the greater the applied energy is, the greater the grain size is up to a certain point. However, once the applied energy exceeds the certain point, the grain size rapidly becomes small to be a microcrystalline. Consequently, laser light source power must be set adequately between a lower limit Ed and an upper limit Eu to obtain sufficiently large grain size (GM). The ELA is required to be always controlled based on the relationship shown in FIG. 1.
Especially when a power setting of the device is largely different from actual effective energy irradiated on the processed film accompanying a degradation of a laser medium, the grain size of the p-Si becomes smaller than that of the intended size in accordance with FIG. 1. Furthermore, in the ELA apparatus, a laser light emitted by a laser source passes through a long distance optical system to make an irradiated form suitable for a predetermined laser annealing. If the optical system is slightly contaminated with moisture, particle or the like at that time, the actual effective energy is lowered.
In addition, dispersion in the actual effective radiation energy is also a problem. In the case that the radiation intensity within an irradiation area of the laser beam is dispersed, the grain hardly grows, especially in an area where the radiation energy is out of the optimum range shown in FIG. 1.
One of the conventional methods for evaluating the grain size of the p-Si is Secco etching. However, a substrate having the film thereon evaluated with such method cannot be used as a product.
Therefore, such evaluation is only for analogizing other substrates.